This invention relates to burn-in and test of electronic devices in surface mount packages. More particularly, it relates to methods and apparatus for temporary mounting and holding electronic device packages for burn-in and/or test and to establishing and maintaining positive electrical contact to closely spaced input/output terminal lands or leads on such packages without damaging the electronic device, the device package, its interconnection terminals or the test socket and without introducing unnecessary signal distortion.
Advances in microelectronics technology tend to develop electronic device chips and packages which occupy less space while performing more functions faster. As a result, the number of electrical interconnections between the device package and external circuitry required for the circuits in the chips to communicate with the outside world increases and the physical size of each such interconnection must decrease.
In order to provide electrical communication between the chip and external circuitry, circuit chips are usually contained within a housing or package which supports interconnection pads or lands, leads, balls, etc., on one or more of its external surfaces. In order to reduce overall lead length from chip to external circuitry and to provide adequate spacing between input/output terminals on the package, high pin count devices are sometimes mounted in packages in which the input/output terminals are in the form of conductive lands or pads formed on one or more faces of the package. The lands are often arranged in rows parallel with and adjacent peripheral edges of one face with the surfaces of the lands coplanar and parallel with (but slightly below) the bottom surface of the package. The lands may be arranged in other patterns such as parallel rows which cover the entire bottom surface in a grid pattern; lands grouped near the center of the bottom surface; or various combinations of such arrangements. Such device packages (commonly known as land grid array or LGA packages) may thus be mounted on circuit patterns on the surface of a circuit board or the like so that the terminal lands are bonded to mating lands or pads on the board.
In some device packages terminal balls may be formed on or substituted for the lands. Terminal balls are usually a quantity of solder which has been heated so that it forms a spherical liquid by surface tension and thus forms a ball-like protrusion extending from the face of the device package. Such device packages (commonly known as ball grid array or BGA packages) may likewise be mounted to circuit patterns on the surface of a circuit board or the like by bonding the solder balls to mating lands or pads on the board.
In many cases it is desirable that the completed device package be subjected to test and/or burn-in prior to acceptance and assembly onto a circuit board. While the terminal lands may be directly and permanently surface mounted on a circuit board by soldering, it is much more difficult to establish and maintain temporary electrical contact with each land without destroying or damaging the land, the package or the encapsulated circuit chip. In order to reliably test and burn-in such packages, the package must be temporarily mounted in a re-useable socket or other mounting apparatus which makes precision interconnection between the input/output lands and outside circuitry without introducing signal distortion problems and without physically damaging the device package.
As the size of the package decreases and the number of lands increases, the size and spacing of lands become smaller. Smaller and more closely spaced lands are, of course, more difficult to contact with test probes or the like. Furthermore, long or massive contact pins cannot be used for connecting external circuitry to the input/output lands for testing when high frequency devices are involved because such contact pins, particularly when closely spaced in order to contact closely spaced lands, introduce unacceptable signal distortion.
Conventional burn-in and test apparatus employs test sockets mounted on a burn-in board with the pin-out leads of the test or burn-in socket passing through the bottom of the socket and through holes in a circuit board in conventional through-hole mountings. Interconnection of high frequency devices with outside circuitry using such conventional mountings can induce unacceptable signal distortion because of the high density of parallel terminal leads passing through the board. More recently, miniaturized surface mount device packages have been devised which have very closely-spaced input/output terminal lands on one face of the package. Such device packages, because of their extremely small size, configuration and physical construction, are most difficult to handle without causing damage, yet good electrical contact with all the input/output terminal lands is essential. It is therefore desirable that apparatus be devised in which such small packages may be easily temporarily mounted (preferably by automation) and tested and/or stress-tested by burn-in and the like without damaging the device package or introducing signal distortion problems.
In accordance with the present invention reliable precise electrical contact is provided between input/output lands on small LGA packages (or balls on BGA packages) and external circuitry by mounting apparatus which employs a support base and contact pins in the form of curved spring fingers which are attached to one face of the support base and extend around the end of (or through a hole in) the support base so that the other end is suspended over the opposite face of the support base. The contact pin is curved in the form of a C to define a curved surface or knee which projects above the opposite face of the support base. The knee is adapted to engage the input/output lands on an LGA package positioned adjacent the opposite face of the support base. The support base is formed of electrically insulating material and carries a set of conductive traces connected to terminal lands which correspond in number and general location with the terminal land arrangement of the LGA package to be tested.
One end of each contact finger is securely attached to the first face of the support base and the spring finger is curved so that its opposite end may only move a very short distance before contacting the opposite face. The contact fingers, however, are precisely aligned with the input/output lands on the device package under test. Since the contact fingers are very short and sharply curved, extremely small diameter wire can be used to form reliably precise contact fingers to contact very small and closely spaced terminal lands. Since the contact fingers are very small and relatively short, they have very little mass. Capacitance-induced, inductance-induced and impedance-induced signal distortion are thus minimized. More significantly, the physical distance (the lead length) between the input/output land and the external circuitry to which it is connected by the contact pin is minimized. Thus the interconnection arrangement of the invention may be used for test and burn-in of extremely high frequency devices in packages employing very closely spaced terminal lands without introducing signal distortion problems. Because of the simplicity of design and operation, the mounting apparatus of the invention may be made from a wide variety of readily available materials. Since the socket is loaded from the top, automated processes may be employed to load and unload the socket without damage to the device package or the socket and the top surface of the device package is exposed for cooling and/or attachment of a heat sink. Because of the simplicity of design, sockets employing the invention may be mass produced quite inexpensively. Other features and advantages of the invention will become more readily understood from the following detailed description taken in connection with the appended claims and attached drawing in which the sole FIGURE is an exploded schematic representation, partially in section, of an LGA device package and mounting apparatus assembly employing the principles of the invention.